1. Field of the Invention
The present invention relates generally to the field of integrated circuits including redistribution layer lines or traces and other interconnect structures and, more particularly, although not necessarily limited to, low profile, packaged integrated circuits including a redistribution layer comprising one or more levels of lines, and methods for their fabrication.
2. State of the Art
In order to function, integrated circuits must be in electrical communication with signal inputs and outputs as well as power and ground or bias connections external to the integrated circuit. For example, power and ground or other reference voltage must be supplied for operation of the integrated circuit, and other connections, such as for input, output and timing signals, may also be required. These connections are typically made through leads or other conductive elements connected to bond pads present on the active surface of a semiconductor die incorporating the integrated circuit.
As electronic devices have become smaller and more sophisticated, the challenge of expanding capabilities while minimizing the space, or “real estate,” also termed the “footprint,” used by an integrated circuit has continued to increase. Techniques for reducing the space used by a semiconductor package include the use of a redistribution layer (RDL) as an additional level of wiring to reposition input and output (I/O) contact locations from the perimeter or center of the active surface to alternative locations. This redistribution process may be necessary if perimeter or central bond pads have to be rerouted into another I/O layout. For example, if the perimeter pad pitch is too fine or does not match the terminal pad layout of a substrate on which the semiconductor die is to be mounted, repositioning may be required. One common example of a redistribution approach is to redistribute a central bond pad layout into a two-dimension array over the active surface of the semiconductor die, the redistributed contact locations then being topped with discrete conductive elements such as solder balls to form a so-called ball grid array.
In a conventional redistribution structure, redistribution layer lines or traces may be embedded into a dielectric material on a surface of a semiconductor die. Typically, the lines or traces are formed on a dielectric layer, and another dielectric layer is then formed over the lines or traces, the ends of the lines or traces at the redistributed contact locations remaining exposed for subsequent disposition or formation of discrete conductive elements thereon. Suitable dielectric materials may include BCB, polyimide, or photosensitive dielectrics. The process employed depends on whether the redistribution lines or traces are formed of aluminum or copper. For aluminum or copper traces, the metal may be sputtered onto the wafer and the traces etched using a photolithography-defined etch mask. In case of copper traces, the metal may be electroplated and then similarly etched to form traces. A layer of dielectric material may then be deposited over and around the traces and apertures formed therethrough to expose the new contact locations. In another approach, commonly termed a damascene process, the lines or traces may be deposited into recesses formed in a dielectric layer. In yet another approach, preformed lines or traces carried on a dielectric film may be applied to the surface of the semiconductor chip. Redistribution lines or traces are typically employed on the active surface of a semiconductor die to enable so-called “flip-chip” mounting of the semiconductor die with its active surface facing the carrier substrate.
For ease of processing, RDLs are conventionally formed simultaneously on a large plurality of semiconductor dice at the wafer level, over the entire surface of a semiconductor wafer. The RDL process is thus performed for every potential semiconductor die in a wafer, including those that may have manufacturing defects and are inoperable or fail to meet desired specifications. Significant materials can thus be wasted in processing useless dice. Further, in order to allow the wafer to undergo the processing needed to form the RDLs, the wafer must be of sufficient thickness to be handled by processing equipment without the risk of damage to the physical structure or integrated circuitry of the wafer. This requirement limits the amount of thinning that can be done to the wafer prior to forming the RDLs. Once RDL formation is completed, thinning the wafer is then constrained by the need to protect the wafer surface from etchants used in chemical thinning, the stress to which the wafer may be subjected without damage during mechanical thinning, known as “back grinding,” or debris created by use of a mechanical thinning process.
Accordingly, a method or system that would be able to effectively form RDLs for semiconductor packages using only semiconductor dice that have been qualified as known good dice (KGD) would be an improvement in the art. Such a technique would enable the fabrication of thin, or low profile, semiconductor packages providing an improvement in the art.